Terbium-activated lanthanum and gadolinium oxyhalide luminescent materials are generally disclosed in U.S. Pat. No. 3,617,743 which includes mention of utilizing said phosphors in a high-pressure mercury lamp. Specifically, a particular phosphor composition containing cerium ion at activator levels was found to emit efficiently under 3650 Angstrom radiation and provide favorable light output maintenance at elevated temperatures.
Fluorescent lamps having an operating temperature range of approximately 100.degree. C and higher are also known, and one such type electrodeless fluorescent lamp is disclosed in U.S. Pat. Nos. 3,500,118 and 3,521,120. Such fluorescent lamps utilize an ionizing energy source providing radio frequency energy to ionize the gas atmosphere contained in a light transmitting envelope. The lamp envelope is also coated on the inner surface with a phosphor coating as the principal means of visible light generation. Conventional type fluorescent lamps wherein the light transmitting envelope is tubular shaped and contains a pair of spaced apart electrodes in each end of the lamp envelope which ionizes a gas filling of mercury and rare gas are also highly loaded and operate at temperatures of 100.degree. C or greater.
It is also known to generate white light in a fluorescent lamp with a phosphor coating comprising a plurality of different phosphor materials wherein each of the phosphor constituents is a sharp line or narrow band emitter having a predominant portion of the emission lying in a relatively narrow region of about 80 nm. These narrow band emitting phosphors are distinguished from wide or broad band emitters wherein the visible emission is so broad that there is no 80 nm. wavelength range where the visible emission is predominantly located. Such known phosphor combinations can be in the form of an admixture employing a narrow band red-emitting phosphor with a narrow band green-emitting phosphor and a narrow band blue-emitting phosphor with the proportions being fixed to generate composite white-light emission.
The predictability of a given phosphor material to produce light emission in a given environment is uncertain. Such unpredictability is especially evident in phosphor combinations being used at elevated temperatures. Maintenance is a critical problem at elevated operating temperatures by reason of additional mercury deposition which occurs and renders efficiency of suitable phosphors as highly speculative. Consequently, suitable phosphors are still being sought for use in fluorescent lamps operating at elevated temperatures which will generate light efficiently and maintain lumen levels comparable to fluorescent lamps being operated at lower temperatures.